Local drug deliveries to the inner ear have become widely used in clinical practice and many new therapies are being developed. However, most of our knowledge of drug pharmacokinetics in the ear is unreliable due to both the technical difficulties in obtaining pure perilymph samples and the highly variable perilymph drug levels produced by round window applications. In the past project period, we developed novel methods of perilymph sampling and highly controlled methods of drug delivery that, when combined, allow meaningful pharmacokinetic studies to be performed for the first time. The experimental studies in our first aim will use cationic and anionic markers to quantify the distribution of substances throughout the cochlear and vestibular systems, following direct injections from pipettes sealed into perilymph at different locations. The role of electrical charge in determining whether substances enter endolymph will be studied. Concentration measurements in vivo will allow the major solute distribution and elimination properties to be quantified and will further validate measurements from fluid sampling procedures. When we understand the basic processes contributing to distribution and elimination from the ear, pharmacokinetic studies of the clinically relevant drugs gentamicin (a cation) and dexamethasone (an anion) will be performed using sequential sampling methods. The pharmacokinetics and pharmacodynamics of the anionic drug salicylate will also be studied, as an agent causing transient sensitivity changes of the ear that can be used as an indirect measure of drug distribution. A second aim will focus on additional factors influencing perilymph drug levels when the drugs are applied intratympanically. This includes study of the rate of drug elimination from the middle ear and how the applied volume affects perilymph concentration. The merits of volume stabilized (gels) or timed release (gel or PLGA nanoparticles) delivery of drugs to the cochlea will be evaluated. In conjunction with these experimental studies, comprehensive 1-D, anatomically based, mathematical models of drug distribution in the fluid and tissue spaces of animal and human ears will be developed. The enhanced models will permit complex experimental protocols to be interpreted quantitatively and will allow realistic prediction of drug distribution patterns in humans. Models will be made available to other groups in the field, permitting quantitative interpretation of a variety of animal and human data. Results from these projects will provide a basic scientific foundation for physiologic studies utilizing drug applications to the ear and will allow the delivery of drugs or other substances to the ears of humans to be optimized for specific purposes.